This invention relates generally to improved methods for disinfecting and/or preserving contact lenses and contact lens care solutions. The solutions contain monomeric biguanides as their prinicpal active disinfecting agent, perform with a low level of residual binding or concentrating onto lens surfaces and exhibit low toxicity levels. The solutions described herein are compatible for use with other germicidal agents for solution preservation, as well as for complimenting or broadening the spectrum of microbicidal activity. In some instances, lower concentrations of such agents may be used, further reducing the risk of residual build-up on soft contact lenses and incidents of eye tissue irritation and inflammation.
Generally, contact lenses in wide use fall into two categories: the hard or rigid corneal type lenses formed from materials prepared by polymerization of acrylic esters, such as polymethyl methacrylate (PMMA), and gel, hydrogel or soft type lenses made of polymerized hydrophilic or hydrophobic monomers, such as 2-hydroxyethyl methacrylate (HEMA). The hard acrylic type contact lenses are characterized by low water vapor diffusion constants, resistance to the affects of light, oxygen and hydrolysis and absorb only minor amounts of aqueous fluids. Because of the durability of hard contact lenses coupled with their tendency not to absorb appreciable amounts of water, the selection of suitable disinfecting agents, cleaning agents or other lens care compounds is relatively non-critical.
However, unlike hard lenses, soft type contact lenses and certain of the newer gas permeable hard contact lenses have a tendency to bind and concentrate significantly more fluids, environmental pollutants, water impurities, as well as antimicrobial agents and other active ingredients commonly found in lens care solutions. In most instances, the low levels of such ingredients in lens care solutions does not lead to eye tissue irritation when used properly. Nevertheless, because of the inherent binding action of protein deposits and soft lens materials disinfecting agents and preservatives tend to build up on lens surfaces, and become concentrated to potentially hazardous levels, such that when released can cause corneal inflammation and other eye tissue irritation.
Previous efforts to alleviate the problem of binding and concentrating disinfectants and preservatives onto contact lens surfaces, and reducing the potential for eye tissue irritation have not been totally satisfactory. For example, in spite of low toxicity levels not all disinfectants are compatible for use with all types of contact lenses. Many hard lens disinfecting and preservative solutions contain benzalkonium chloride or chlorobutanol. Although they are effective antibacterial agents, their use can result in a loss of lens hydrophilic properties, cause solution instability or may even lack compatibility with certain types of hard lenses, e.g. high silicon content.
Other antibacterial agents were found to be more compatible with contact lenses and exhibit less binding on lens surfaces. U.S. Pat. No. 4,361,548 discloses a contact lens disinfectant and preservative containing dilute aqueous solutions of dimethyldiallylammonium chloride (DMDAAC) wherein amounts of DMDAAC as low as 0.00001 percent by weight are employed when an enhancer, such as thimerosal, sorbic acid or phenylmercuric salt are used therewith. Although lens binding and concomitant eye tissue irritation with DMDAAC were reduced, it was found in some users to be above desirable clinical levels.
Other efforts to reduce or eliminate soft lens binding have led to the use of anti-binding or detoxifying agents, like polyvinyl pyrrolidone (PVP) and polyvinyl alcohol (PVA). However, these polymers alone were found to be ineffective, for the most part, in reducing lens binding and eye tissue irritation.
Heretofore, monomeric biguanides have been used in lens disinfecting and preservative solutions. For example, the biguanide chlorhexidine was reported by B. S. Plant et al in the J. Pharm. Pharmacol., 32, 453-459 (1980) suggesting its use as a disinfectant with soft contact lenses. It was discovered, however, that chlorhexidine was absorbed to some extent by such lenses and was therefore not an ideal agent for such systems. One solution to this problem is suggested by U.S. Pat. No. 4,354,952 which discloses very dilute disinfecting and cleaning solutions containing chlorhexidine or its salts in combination with certain amphoteric and non-ionic surfactants. Although these solutions are effective in reducing binding properties, their antimicrobial activity may be diminished when used with certain amphoteric surfactants. In the case of chlorhexidine, it was found that if not used in the proper ratio the surfactant and disinfectant will precipitate unless a combination of surfactants are employed.
While it has been reported that more conventional chlorhexidine-containing disinfectant solutions bind to soft contact lenses seven times less than benzalkonium chloride, the presence of proteinaceous oily, tear-film deposits can double the amount of chlorhexidine absorbed over that of clean lenses.
Other efforts to alleviate the problem of contact lens binding and eye tissue inflammation are disclosed in copending U.S. application Ser. No. 528,322, filed Aug. 31, 1983. There, water soluble polymeric biguanides having molecular weights of up to 100,000 were found to demonstrate an unusually low level of absorption and residual build-up on contact lens surfaces. It was discovered, for example, that polyhexamethylene biguanides could be used at extremely low levels in contact lens disinfecting solutions while providing a broad spectrum of microbicidal activity, substantially free of binding effects and without prompting a cytotoxic response.
It was also quite surprising to have found that not only did the polymeric biguanides demonstrate unusually low levels of lens absorption, binding and toxic response, but the monomers of such polymers also performed comparably. This was rather unexpected in view of past efforts to solve the aforestated problem have generally shown that polymers, and usually higher molecular weight materials, provided the lowest levels of lens absorption and binding activity. That is to say, heretofore, it was generally held that polymeric type disinfectants offered the most probable rationale for solving the problem of lens binding and eye tissue irritation, since their molecules were less likely to penetrate lens micropores than their monomeric counterparts. Hence, the present invention provides for an improved and unexpected new means for treating contact lenses. The solutions are compatible for use with both hard and soft type lenses, and are adaptable for use with virtually any of the commonly known disinfecting techniques, including "cold" soaking under ambient temperature conditions, as well as with high temperature disinfecting methods. The disinfecting and preservative solutions employed according to the methods described herein are especially noteworthy for their wide spectrum of bactericidal and fungicidal activity, at low concentrations coupled with low toxicity levels and reduced affinity for binding and concentrating when used with soft type contact lenses.